14C, a proxy of ‘biomass burning’ versus ‘fossil fuel combustion’ contribution to carbonaceous aerosols

Carbonaceous aerosols (organic and elemental carbon) in the atmosphere are produced by biomass burning and fossil fuel combustion but their relative contribution is not properly known. Radiocarbon (14C) is present in living and recently living material at an approximate concentration of one 14C atom per 10^12 ordinary carbon atoms (12C + 13C). This equilibrium amount is a result of the gain of 14C from its steady production by cosmic rays spallation reaction with nitrogen in the atmosphere (some fraction of which is taken up by the biosphere through photosynthesis in the form of 14CO2) versus the loss of 14C from its radioactive decay (5730 y half-life). However, 14C is absent from fossil fuels because of the ancient age of fossil carbon (due to radioactive decay to unmeasurably small amounts). This dichotomy is the basis of inferring the fraction of fossil carbon in an ambient aerosol sample by comparing its 14C content to that of living material, a methodology that has become increasingly popular in recent years. For detail, please see Charles et al (2006) and references therein.

Reference:
Charles W. L., John, V., James, N. B., William S. C., William A. L., and Ann P. M., 2006, Absence of 14C in PM2.5 Emissions from Gasohol Combustion in Small Engines, Aerosol Science and Technology, 40:657–663.